In numerous applications there is a need to perform beamforming operations to acquire spatial information regarding a particular region of interest. Various systems have been developed to perform such beamforming operations which frequently depend upon the particular applications.
One application involves beamsteering and/or beamforming in medical ultrasound systems used to image internal organs. For undersea acoustic mine-field reconnaissance and mine hunting applications, high-resolution imaging sonars are needed for clutter rejection, obstacle avoidance, and identification of possible mines. For these applications, a light-weight, low-power, low-cost and portable sonar system is needed for use in a diver""s hands, in a remote imaging sonar on an unmanned undersea vehicle, or with a small surface-craft sonar.
The present invention relates to a high-resolution, three-dimensional imaging system based on a large-area electronically steerable two-dimensional array. The large aperture provides a beamwidth less than 0.3xc2x0. Beamforming, or beamsteering, can be performed using time-domain delay-and-sum operations. A delay-and-sum beamformer allows a 2D array to xe2x80x9clookxe2x80x9d for signals propagating in a particular direction. By adjusting the delays associated with each element of the array, the array""s directivity can be electronically steered toward the source of radiation. By systematically varying the beamformer""s delays and its shading along a 2D imaging plane, a 2D scan response of the array can be measured and resulting 2D images representing the 3D radiation sources can be created. The proposed system can provide continuous real-time 128-by-128-pixel scanned images throughout a 14xc2x0 field of view. The proposed delay-and sum beamforming approach allows target range information to be obtained from the time-of-flight calculations. When a target area is identified by the proposed electronically steerable sonar system, the beamforming electronics can be adjusted to zoom-in to a smaller field-of-view for high-resolution imagery.
Components of the system include a large-area, low-insertion loss and high-bandwidth sparse array, a 32-channel beamforming processor, a bank of low-noise amplifiers, and CDP FIR filters for baseband signal conversion. The beamforming electronics and FIR filters use low-power, high-throughput charge-domain-processing (CDP) technology. At a 40 MHZ clock rate, the beamformer can provide a continuous computation throughput of 10.2 billion operations per second and a delay update rate of 28 billion bits/s and dissipate approximately 1 W of electric power. A technology comparison (shown in Table 1) of the present low-power, 2D beamformer, a commonly-used microprocessor and a digital signal processor (DSP) the Texas Instruments"" TMS320C6201 processor, demonstrates the more than an order-of magnitude power savings offered by a preferred embodiment of the invention.
In a preferred embodiment of the invention uses a shading procedure to increase the ratio of the main lobe relative to the side lobes to provide a difference of at least 20 dB, and preferably of at least 25 dB to provide the desired image quality. The small size of the array, such as a 10 cmxc3x9710 cm system, or alternatively a 20 cmxc3x9720 cm system, can provide a low power, high resolution system suitable for use by divers or small vessels, for example.